1. Field of the Invention
This invention relates to a circuit for comparing analog signals and, more particularly, to a circuit for determining the analog signal of the lesser potential.
2. Description of the Prior Art
Analog devices continue to find a wide variety of applications. The theoretically infinite resolution and the continuous availability of output signals are useful characteristics promoting the use of analog devices. Analog devices may be fluidic or electrical. In fluidic devices the physical continuity of the fluid lends itself to continuous control as provided by analog devices. In electrical devices, the inherent analog character of transistors facilitates the development of analog devices.
One application in which analog devices find utility is a closed-loop positioning system used, for example, in heating and air conditioning systems. In such systems a monitoring device such as bimetallic elements or diaphragm continuously monitors a selected characteristic of the system such as temperature or pressure. The monitoring device generates an analog signal representing a desired position of a motor shaft while another device coupled to the motor shaft generates an analog signal representative of the actual position of the motor shaft. If the desired and actual positions of the motor shaft do not correspond, an analog signal comparator detecting the difference generates a signal for energizing windings of the motor to rotate the shaft to the desired position. Often a motor shaft must be rotatable in both clockwise and counter-clockwise directions. Accordingly, some prior art motor actuator circuits have used electromechanical balance relays which sense both the amplitude and direction of the motor shaft excursion. However, switching the large inductive currents associated with electric motor windings causes undesirable arcing and burring at the relay contacts. Moreover, many relays are subject to power fluctuations and fatigue fractures.
To overcome the problems associated with relay motor controls as well as to improve the performance of a motor control, the inventor earlier made an invention disclosed in U.S. Pat. No. 3,742,327 issued June 26, 1973. Although the invention disclosed in the above identified patent is fully adequate for the purpose intended, the disclosed circuitry is somewhat complex and, therefore, expensive.